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[fwd]Why I Chose Erlang

发表于: 2006-11-13   作者:cookoo   来源:转载   浏览:
摘要: Why I chose erlang (very, very long story) http://www.erlang.org/ml-archive/erlang-questions/200302/msg00013.html I've been following the discussions about OTP the past few days and found the c
Why I chose erlang (very, very long story)

http://www.erlang.org/ml-archive/erlang-questions/200302/msg00013.html

I've been following the discussions about OTP the past few days and found
the comments by various participants particularly enlightening. It's
always nice to observe a lively discussion -- so much can be learned. I
thought I'd throw in my opinions as to why I am using erlang. I hope they
are useful to someone, especially the Ericsson Product Manager.

My background is rather varied having started with Basic, assembler and
RPG, before moving on to APL, Prolog, Pascal, Forth, Smalltalk-80, Modula-2
(on the Lilith), then those halcyon days of Symbolics LISP with Flavors &
Z-macs (an editor without meta-, hyper-, and super- seems so limited),
Expert Systems and other AI techniques, a rude introduction to C on Sun
(why is this new concept of memory allocation necessary?), followed by C++,
OODBs, CLOS with MOP (MOP being the main reason why OO was so interesting
in the first place -- why, oh why, was it eliminated in future OO
languages, just to allow mere mortals with no passion for programming to
have access to the Gospel?), Perl, Javascript, Java, Python, Ruby (now that
is a fine OO language for small problems) and now erlang.

I am first and foremost a language dilettante, but when I find one that is
useful I work with it to discover what is different about it and in what
situations it might apply. I have written small things (compared neural
net implementations in Pascal, spreadsheet macros and APL, a mini-SECD
machine in assembler on TRS-80) to large things (traditional
business/banking/manufacturing inventory/accounting/personnel systems,
aerospace message-based OODB applications, a Perl program that
automatically loaded a structured database from English text free-formatted
OCRed resumes) to Internet things (webserver / gameserver, meta-search
tools, html generators, data extraction systems). Unfortunately, a job
often dictates the language and the problem so my useful efforts have been
limited.

My observations about languages:

1) Spend a little time evaluating and toss quickly those that you don't like
2) Toy problems are designed to make a language look good or to point out
its strongest features
3) Fiddling with your own toy problems highlights issues of a language
relevant to your use of it
4) Until you implement a real problem you can't understand the language
5) Until you hit the limits of applicability you aren't proficient in a
language
6) A language will subliminimally and fundamentally alter your perception
of a problem
7) Don't believe the hype
8) The proper or improper design of data structures can make a problem
range from trivial to unsolvable; ease of expressing and redesigning data
structures is essential to solving real problems

#6 is most apparent if you compare any combination of a functional
language, imperative language, constraint-base language, rule-based
language, OO language, dataflow lnaguage, pattern-based language or any
other paradigm to the same problem. Another easy way to see #6 is to
discover the right language for a problem, solve it elegantly and then try
to talk to a C, C++, or Java programmer about the problem.

I have always been partial to recursive languages, functional languages,
and pattern-based languages (I even played around a little with REFAL on
that last note). A combination of these attributes are more natural and
readable to my sensibilities. Others may have their own conceptual and
readability biases.

As an aside: I hate statically typed languages, and so-called OO languages
where the data have no type, but the containers you put them in have type
and forces same on its data. I never understood the usefulness of a
language that dictates if you take your laptop to the library it magically
becomes a book and has no notion that it was ever anything else.


What attracted me to erlang:

I have been building a webserver / gameserver for the last 4 years on and
off in my spare time (see www.5x5.net) out of general curiosity to
understand all aspects of eCommerce from the technology underlying the
gamesite, to the marketing, advertising, and business implications of an
Internet only company, but also because I saw that the mega-media companies
were taking over the Internet and service providers and I wanted to have my
own capability that couldn't be usurped by the software vendor or service
provider. At the time I thought Java was mature enough to support what I
wanted to do, but it did have a couple limitations, including the fact that
each simultaneous player had to have a separate thread (no support for
non-blocking threads). I thought this would be challenging and teach me a
little about threaded programming, but without the headaches of a C or C++
approach.

After about 30K lines of Java in both applets and Tomcat servlets I was
nearly ready to launch the website. I had built all the user account
management, registration, demographics, automated email, MySQL web-based
management tools (so I could manage the site from work or from home), and
the gameserver (designed to deliver HTML pages, applets and ads all from
the same IP address and port to tunnel through firewalls and deliver ads
directly to the applet), and started an Alpha test. After 3-4 days the
thread that reports when people enter and leave game rooms stopped
responding. It didn't crash but it didn't do any more work. All other
threads worked fine with ads, webpages and HTML still being
delivered. Somehow it starved.   After 4 weeks or so, Tomcat stopped
running servlets intermittently. The logfiles had a message that the JVM
could no longer spawn new threads (is that Java, Linux or Tomcat with the
problem?).

I had built some kickstart scripts and cronjobs that would attempt to
restore freezes, etc. (oooh, this sounds too much like Microsoft and IBM's
"self-healing" software), but it all seemed so hokey and unnecessary -- and
still didn't solve the problem of giving me a remotely administerable
non-stop server. Whenever the server came down all users in the gamerooms
would freeze or get booted from their applets. There was no redundancy,
distributed processing or fault tolerance. The latter was my mistake in
the message protocol because it was possible to lose one message and not
know the state of the game or be able to rejoin and reproduce without
leaving the table and disrupting it for everyone else. I learned quite a
bit, including Visual Age Java, but only enough to believe that while Java
could solve the problem, it was the wrong language -- I felt I had hit its
limits for this application. In desperation I abandoned the project and
started browsing the web listlessly for several months.

The main issues with Java boiled down to:

1) Threads are much easier than C, but still a royal pain
2) There are no useful tools for administering a server that I built
3) The OO straight-jacket is a major hindrance to web-based computing
4) There is no fault tolerance or distributed computing, the VM is heavy
and flaky
5) The use of Java applets put a heavy constraint on the server in
delivering content
6) Performance was always a concern, especially handling thousands of threads
7) Embedded capabilities were just being developed and not stable or
performing well

When I came across erlang it was obvious that #3 was solved straight
away. The biggest pain was in message passing. Messages had to be
received as a generic Msg type, a jump table based on the first byte was
used to get a msg handler, which in turn parsed, converted and cast the
message as the right object type (hopefully). Fighting the static type
checking of the compiler every step of the way, I successfully created
type-less objects and a functional callback implementation that resulted in
a remote object (which of course had no knowledge or state corresponding to
the originating object's environment space). All this immediately
disappears and becomes natural with a functional approach, the first
solution that you would attempt rather than the last futile try.

Threading seemed to be completely a non-issue in erlang, with the added
benefit that every user could be a separate process in addition to the game
table itself, leading to an automatic distributed, fault-tolerant approach
of losing only one game room rather than the entire server in a failure
situation (and even the possibility that a game room fails but the game may
continue on a new instance of a game room). I could freely add servers to
allow more players without any more effort than the load-balancing
aspect. The lighter VM solved #4, and the lightweight processes solved #6
beyond my wildest dreams (probably even beyond my ability to design a
meaningful UI for 50,000 game tables per server).

#5 was now not as great a concern because the load-balancing reduces the
strain created by the Java applets. The trick is just faking them into
believing everything is coming from the same server, or transparently
relocating the user before delivering the applet.

I have been reading the docs, fiddling around and have started
reimplementing the server. I am getting 5-10 times code reduction with an
incredible increase in code clarity. HTML and the pattern-based functional
style are the perfect marriage. This brings back the happy days of LISP,
but I find the code much more readable and the language concepts more
succinct for this problem set than LISP was. The beauty of erlang is that
it is small, clear and succinct and therefore implements problems clearly
and efficiently.

I've spent the last 4 months or so following the email list and reading
docs. The learning curve for the full system is steeper than most every
language I've pursued except the most interesting/useful ones:
Smalltalk-80, Common LISP Object System with the Meta Object Protocol, and
Java. Java may prove to be the largest and least useful of the set, its
usefulness mainly is in improving on the C++ experiment. (Ada has to be
the absolutely least useful steep learning curve language because it was
designed by committee and not to solve any particular problem). The
learning curve on the language itself is practically nil if you've ever
used a functional language before, and if you haven't erlang promises the
greatest hope that you ever will.

OTP. The raging debate about this was the most informative thing I've read
over the last few months, aside from those postings that corrected all my
misperceptions of erlang the language. I decided to invest the time in
erlang rather than skip over it like I did Mozart OZ, precisely because OTP
was present. I have never seen anything like it, and it solved exactly the
problem that I did not want to waste my time on -- developing another Java
/ Linux application set for managing the server. That task would have been
bigger than the gameserver itself that had taken me way too long already.

I believe OTP provides me:

1) Embedded turnkey system using SASL, reboot and all that
2) Non-stop server using supervision trees
3) Failover and takeover for seamless, continuous gameplay during server
maintenance
4) Distributed debugging and logging that can be turned on and off at will
5) A complicated build, but with the result of hotcode version upgrade
6) Mnesia distributed database, with in memory and on disk capabilities
7) Pre-built behaviours that match my problem set

Maybe I'm mistaken in evaluating based on the documents, and I'll find out
that I'm not capable of making OTP work, but it has more promise than
anything I've seen. OTP solves problems that I've not seen examples of
having been solved before. I'm learning and the best way to learn is to
take a real problem, push the limits and see where things break. In 6
months I should know. When I'm done with all this, I expect to be able to
have gained knowledge enough that when someone asks me "Can A or B be
done?" I can reply based on first-hand experience and have the confidence
that if I say yes, I would be capable of delivering what was asked. In 5
years I may have to roll my own OTP; maybe I'll be deep in the source code
6 months from now figuring out how to roll my own. If I was still stuck
with Java, I would be spending my time at the beach instead.

Besides all that, erlang has got me excited about programming again. I'm
trying to figure out how to sneak a prototype into my job to convert them
as well. Right now, my day job is programming stock trading systems for
mutual fund administrators using a homebrew rule-based dataflow language
that no one seems to want any more (the language was developed there long
before I arrived). Guess what, the problem requires non-stop, distributed,
fault-tolerant, constraint-based, functional capabilites, with in memory
DB, distributed logging, failover and realtime interactive monitoring,
querying and progress reporting. Sounds like a job for erlang and OTP
(Online Trading Platform). As a programmer, I really could care less what
it is called, does it solve my problem in a way that I enjoy coding and
maintaining?

P.S. The Telephony of OTP did really put me off and delayed my looking
into the capability until my problem made it apparent this was really
useful. From a marketing standpoint a name is needed, as well as a
separation between the language and the platform. More would download and
play with the language. Only those with a real need will download the
platform.


Sorry for the loss of bandwidth. Had to drop in my 20,000 drachmas worth.

jay

[fwd]Why I Chose Erlang

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